Several components of a motor vehicle powertrain may employ a wet clutch to facilitate the transfer of power from the vehicle's power generator (i.e, an internal combustion engine, electric motor, fuel cell, etc.) to the drive wheels. A transmission located downstream from the power generator and which enables vehicle launch, gear shifting, and other torque transfer events is one such component. Some form of a wet clutch may be found throughout many different types of transmissions currently available for motor vehicle operation. A wet clutch may be utilized in a torque converter for an automatic transmission, a multi-plate wet clutch pack for an automatic transmission or a semi-automatic dual-clutch transmission (DCT), and a wet start clutch that may be incorporated into a sportier automatic transmission equipped with as many as seven to nine gears as a substitute for the torque converter, to name but a few examples. Similar wet clutches may be found elsewhere in the vehicle powertrain besides the transmission.
A wet clutch is an assembly that interlocks two or more opposed, rotating surfaces in the presence of a lubricant by imposing selective interfacial frictional engagement between those surfaces. A friction clutch plate, a band, a synchronizer ring, or some other part that provides one of these engageable rotating surfaces typically supports a friction material to effectuate the intended interlocking frictional engagement. The presence of the lubricant at the friction interface cools and reduces wear of the friction material and permits some initial slip to occur so that torque transfer proceeds gradually, although very quickly, in an effort to avoid the discomfort that may accompany an abrupt torque transfer event (i.e, shift shock). But maintaining the lubricant at the friction interface has an adverse impact on fuel efficiency. This is because the power needed to pump the lubricant, usually under pressure, to and from the friction interface at a flow rate that keeps the surface of the friction material below a certain temperature is ultimately siphoned from the power generator.
Conventional friction materials generally cannot function reliably at surface temperatures above 300-350° C. Above those temperatures, such friction materials tend to suffer from lubricant thermal degradation and glazing—a process in which the surface of the friction material accumulates thermally degraded lubricant additives to form a substantially impenetrable sludge deposit. A surface-glazed friction material may contribute to a variety of complications including shuddering and an inconsistent coefficient of friction across the friction interface. Exactly how much lubricant needs to be circulated over the friction material to sustain a low enough surface temperature depends on the configuration of the wet clutch and the surface area of the friction interface between the opposed, rotating surfaces.